Modern automotive engine controls rely predominantly upon two operating control modes. One control mode generally corresponds to engine speed (RPM) and is referred to herein as RPM mode control. Such control is characteristically invoked by minimal or no throttle pedal depression and is generally characterized by significantly throttled engine operation, e.g. low load engine idle. The other control mode (torque mode) generally corresponds to varying degrees of engine throttling in accordance with varying operator demands such as through throttle pedal depressions and vehicle cruise control settings. This latter mode also includes scenarios of controlled vehicle coastdown which indirectly limits the engine braking effect upon the vehicle by controlling the throttle angle subsequent to throttle pedal release by the operator or a canceled cruise setting, for example.
The controls for both such modes are known to provide a resultant desired mass airflow signal which is utilized by an airflow converter to establish throttle valve/idle air control valve commands as appropriate in accordance with the particular engine control hardware. In some engine systems a throttle valve alone provides idle air flow whereas in other systems an idle air control (IAC) valve provides a measure of throttle valve bypass airflow.
It is known to satisfactorily control the transition from torque mode to RPM mode. In mechanically linked throttle systems the throttle return to an idle position upon release of the accelerator pedal by the operator is damped. This improves coastdown feel for the operator and also prevents the engine from stalling, particularly under heavy loads conditions. In so called throttle follower systems, an IAC valve is open slightly at very low throttle angles and opens up more as the throttle opens up. In such systems, when throttle position decreases upon the release of the throttle pedal the IAC motor will close slowly to effect the desirable coastdown feel and anti-stall benefits described.
Typically in modern engine controls, a control command is periodically adjusted to provide for engine output torque requirement. Commonly, the control command is directed to an engine intake air rate control actuator, such as the throttle valve and IAC valve as the case may be, to vary engine intake air rate to achieve the output torque requirement during RPM and torque modes. Commonly, the torque requirement is established in accordance with stored calibration information resolved during a conventional calibration process from such engine parameters as intake manifold absolute pressure, engine speed, spark timing advance, air/fuel ratio, ambient temperature, engine coolant temperature, and barometric pressure.
There exists, however, certain torque discontinuities in the transition between RPM mode and torque mode. Such discontinuities may be result in noticeable driveline disturbances that are objectionable to the vehicle operator, engine speed sags, or throttle response characteristics which are not pleasing to the driver. Therefore, there is a need for an improved means of transitioning from RPM mode to torque mode.